The invention is directed to an at least partially striking electrical hand operated tool driving device, such as a hammer drill or a chisel hammer, with a no-load stroke disconnection of an air striking mechanism.
For producing the impact, electrical hand operated tool driving devices with air striking mechanisms make use of a back and forth oscillating movement, which is applied by an air spring between an energizing piston, movable in a hollow cylinder relative to the cylinder and sealing it, and a spaced free piston, which can be moved, forming a seal, in the direction of the tool axis and axially strikes the tool over an anvil. Advantageously, in the absence of a counter-pressure, the impact on the tool, which can move axially to a limited extent in a tool seat, is interrupted over a no-load stroke disconnection.
According to EP759341A3, when the anvil assumes a no-load stroke position, made possible by an enlargement of the no-load path, the air spring is vented to the ambient atmosphere or the control valves for the no-load stroke disconnection and, with that, a pressure difference, producing the impact, is prevented. The valves and valve control, required for this, are a disadvantage of such a solution.
For disconnecting the no-load stroke, in accordance with WO88/06508, the driving mechanism of the striking mechanism is interrupted automatically over a coupling, when the tool assumes an axial no-load stroke position, which is measured with a sensor and evaluated by a control unit. The coupling, required for this purpose, is a disadvantage of such a solution.
It is a primary object of the invention to provide a simple solution for the no-load stroke disconnection of an at least partially striking electrical hand operated tool driving device.
Essentially, the axial position of the tool, the anvil or the striking mechanism is scanned with a sensor and this information is evaluated by a control unit which, in the event of a no-load stroke, lowers the rpm of the energizing driving mechanism to a previously fixed value. Accordingly, the invention holds, according to the law of cause and effect, that the rpm of the energizing driving mechanism, which causes the no-load strokes, remains constant. When the rpm of the energizing driving mechanism has been lowered sufficiently, a pressure difference, accelerating the free piston, is no longer developed due to the venting openings, as a result of which no further impacts on the anvil and the tool are produced. For this purpose, the rpm of the energizing driving mechanism is reduced to about 10 to 50% of the working rpm.
Advantageously, the sensor is an inductive or capacitive sensor, which responds to the axial position of one of the tool, the anvil or the striking mechanism.
Alternatively, the sensor is a Hall sensor, which is assigned to a magnet, and can be moved relative to the sensor and is connected with an actuator, which reads the axial position of the tool, the anvil or the striking mechanism.
Preferably, this actuator is constructed fork-shaped, engaging the anvil axially, and fastened at the outer periphery of the anvil, for example, with a light aluminum ring. The actuator, advantageously consisting of a light, very stiff, fiber-reinforced plastic, has a small magnet, which moves the Hall sensor together with the anvil.
Advantageously, the energizing driving mechanism is constructed as an electric motor, the rpm of which can be reduced electronically over the motor electronics by the control unit.
Further, the energizing driving mechanism and an optimum rotational driving mechanism are provided by a common electric motor over a transmission, as a result of which, in the case of a no-load stroke, the rpm is also decreased by an optional rotational movement.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by it use, references should be had to the drawings and description matter in which there are illustrated and described preferred embodiments of the invention.